The embodiments described herein relate generally to combined cycle power generation systems and, more particularly, to methods and apparatus that facilitate starting such systems.
At least some known combined cycle power systems include one or more gas turbines, at least one heat recovery steam generator (HRSG), and at least one steam turbine. The HRSG and the steam turbine are coupled in flow communication via steam piping. Sufficiently warming the steam piping by channeling steam from the HRSG to the steam turbine facilitates attaining the proper steam conditions at an inlet to the steam turbine. Known combined cycle system startup procedures require steam produced from the HRSG to initially bypass the steam turbine until pressures and temperatures are within predetermined superheat parameters or within ranges suitable for admitting steam to the steam turbine.
During this bypass operation, a section of steam piping between the bypass line and closed steam turbine inlet isolation valves does not receive warming steam flow. Therefore, warming this section of steam piping to the predetermined temperature range cannot be facilitated by the bypass operation. These dead-headed sections are filled with non-moving, or stagnant, fluid that cools to temperatures below those parameters suitable for steam turbine admission. Therefore, upon completion of bypass operations, this section of steam line needs to be heated to a range within those predetermined parameters prior to steam admission to the steam turbine. Heating this section of steam line subsequent to the bypass operations delays steam admission to the steam turbine. This delay may exceed 20 minutes, thereby further extending the overall combined cycle start time. Moreover, the length of piping with stagnant fluid therein could be significant, thereby extending the delay associated with warming up the associated sections of the affected piping. Furthermore, such delays facilitate additional fuel consumption, thereby decreasing the efficiency of the combined cycle power systems and increasing emissions therefrom. Furthermore, admission of hot, live steam to sections of piping containing stagnant, relatively cold fluid may induce additional stresses in the associated steam piping, thereby facilitating a reduction in an expected lifetime of the piping.
Some known combined cycle power systems have used such traditional starting procedures at least in part because such systems were based-loaded and startups may have been infrequent. Moreover, there may have been no sense of urgency because any known startup delays would be taken into account during the infrequent startups. However, with increased and more frequent day-to-night power price swings, such unplanned startups have become more frequent. For example, there is an increasing trend to use combined cycle power plants as daily peaking units because of the periodic changes of electric power demand and fuel prices. Also, some renewable energy sources, for example, wind turbines, may experience sudden reductions in wind, thereby facilitating unplanned starts of combined cycle power systems. The steam line heating delay described above may decrease timely commercial delivery of electric power generation from the combined cycle power systems.
Some known combined cycle power systems include additional piping and valving positioned closer to the steam turbine inlet isolation valves. The additional piping and valving is costly to install, is costly to maintain, facilitates additional points of failure during startups, and is difficult to install during steam turbine retrofits.